To increase safety, most modern car brake systems are broken into two
circuits, with two wheels on each circuit. If a fluid leak occurs in
one circuit, only two of the wheels will lose their brakes and your car
will still be able to stop when you press the brake pedal.

The master cylinder supplies pressure to both circuits of the
car. It is a remarkable device that uses two pistons in the same
cylinder in a way that makes the cylinder relatively failsafe. The
combination valve warns the driver if there is a problem with the
brake system, and also does a few more things to make your car safer to
drive.

In this article, we will learn how the master cylinder and
combination valve work.

The
Master Cylinder
Here is where you'll find the master cylinder:

Master cylinder location

In the figure below, the plastic tank you see is the
brake-fluid reservoir, the master cylinder's brake-fluid source.
The electrical connection is a sensor that triggers a warning
light when the brake fluid gets low.